Once-through steam generator for burning dry brown coal

ABSTRACT

The invention relates to a forced-flow steam generator for burning dry brown coal without assistance from recirculated waste gas in the combustion chamber thereof. Said forced-flow steam generator ( 1 ) comprises a combustion chamber ( 2 ) and a waste gas flue ( 3 ) connected to the upper end thereof, and peripheral walls ( 4 ) surrounding said flue. Said walls ( 4 ) are formed from tubular walls ( 5 ), the tubes thereof guiding the water/steam working medium. The combustion chamber ( 2 ) comprises at least one burner ( 6 ), and downstream heating surfaces ( 7 ) are arranged in the waste gas flue ( 3 ). Part of the peripheral walls ( 4 ) is covered in the region of the combustion chamber ( 2 ) by at least one bulkhead heating surface ( 8 ), the size of which on the surface side being determined such that the heat absorption of the peripheral walls ( 4 ) and therefore the temperature thereof are reduced to a value enabling the formation of the peripheral walls ( 4 ) from modified, heat-resistant 2.25-2.5% chrome steel that does not require any heat aftertreatment following the welding treatment thereof.

The invention relates to a once-through steam generator for burning dry brown coal without the aid of recycled flue gas in its combustion chamber, the once-through steam generator having a combustion chamber, a flue gas pass which adjoins the upper end of said combustion chamber, and enclosure walls which enclose them, the enclosure walls being formed from tube walls, the tubes of which conduct the working medium water/steam, the combustion chamber having at least one burner, and heating surfaces being arranged in the flue gas pass.

Once-through steam generators are known from the publication “Kraftwerkstechnik” [Power plant technology], Springer-Verlag, 2nd edition 1994, chapter 4.4.2.4—Forced flow (pages 171 to 174), Prof. Dr.-Ing. Karl Strauβ, and are used in power plants to generate electric energy by burning, for example, fossil fuels. In a once-through steam generator, the heating of the tube walls or enclosure walls which form the combustion chamber or the gas flue leads, in contrast to a natural circulation or forced circulation steam generator with only partial evaporation of the water/steam mixture conducted in the circulation, to an evaporation of the flow or working medium in the tubes of the tube walls or enclosure walls in a single throughflow.

A once-through steam generator which is fired with dry brown coal (DBC) or a power plant which is configured with said steam generator has considerable potential to increase the degree of efficiency over a once-through steam generator which is fired conventionally with raw brown coal (RBC) or a power plant which is configured with said steam generator. Here, the DBC is produced in an energetically favorable manner from RBC in a process which takes place before the combustion. As a result of the reduced water content of DBC compared with RBC, the calorific value and the heat quantity to be transferred thereby in the combustion chamber of the steam generator rise considerably. In accordance with the ash properties of the RBC or the DBC, the combustion chamber has to be designed in such a way that a final combustion chamber temperature in the range from 950 to 1150° C. is set by the heat absorption of the enclosure walls of the combustion chamber. The medium-side steam temperature which results on account of the heat absorption of the enclosure walls is pivotal for the material selection of the tube walls which are used as enclosure walls and are produced from a tube-web-tube connection. In once-through steam generators with high, supercritical steam parameters, a defined part of flue gases has to be recycled into the combustion chamber (flue gas recirculation) when firing DBC in order to reduce the heat absorption of the enclosure walls in the combustion chamber, in order that the highly loaded parts (oblique winding and perpendicular bore) of the enclosure walls which are configured as tube walls can be formed from the special materials T23 (a material which is approved by the ASME (American Society of Mechanical Engineers)), T24 (7CrMoVTiB10-10) or other materials with a similar chemical composition which all belong to the category of modified, heat-resistant 2.25-2.5% chromium steels. These materials have the advantage that they are especially suitable for high steam parameters and that they can be welded without thermal post-treatment, and therefore the production of the enclosure walls or tube walls and their assembly on the construction site can be carried out simply. The material T23 is listed, for example, in the VdTÜV material sheet 511/2, edition 06.2001 and the material T24 is listed, for example, in the standard specification sheet DIN EN 10216-2, edition October 2007.

In the case of the use of flue gas recirculation, a flue gas recirculation system with a flue gas recirculation fan is necessary. The convection heating surfaces which are used are in some circumstances larger than if no flue gas recirculation is used. The flue gas recirculation system and the additional heating surfaces represent high investment costs. In a further disadvantageous way, the flue gas recirculation system increases the internal electric consumption of the power plant and increases the running operating costs.

As has already been mentioned above, natural circulation and forced circulation steam generators are also known in addition to the once-through steam generators. In these known recirculation or circulation steam generators with subcritical steam parameters, only a defined quantity of heat can be absorbed by the evaporator of the steam generator. Said quantity of heat is defined by the operating pressure of the plant. As a result of the properties of the fuel (ash composition, fusion properties of the ash), the quantity of heat to be transferred from the combustion in the firing chamber or in the combustion chamber can be higher than that which can be absorbed by the evaporator. Platen heating surfaces have then been used in circulation steam generators, in order to limit the quantity of heat to be transferred to the evaporator and/or to emit the excess quantity of heat to the platen heating surface. In contrast to the natural circulation and forced circulation steam generators, the heat absorption is not limited in a once-through steam generator in its evaporator, since the medium temperature at the evaporator outlet is already superheated during once-through operation and the level of the superheating can be fixed variably. The associated temperature level of the steam or the corresponding design temperature in the enclosure walls is controlled by a suitable material selection and, if DBC is burnt, by suitable flue gas recirculation into the combustion chamber.

It is then an object of the invention to provide a once-through steam generator for burning dry brown coal, in which once-through steam generator the abovementioned disadvantages are avoided or in which once-through steam generator, if dry brown coal is burned, the use of flue gas recirculation is avoided and the medium temperature in the enclosure walls or the tube walls is not increased in comparison with the once-through steam generator fired by raw brown coal.

The abovementioned object is achieved by the entirety of the features of patent claim 1.

Advantageous refinements of the invention can be gathered from the subclaims.

The solution according to the invention provides a once-through steam generator for burning dry brown coal, which once-through steam generator has the following advantages:

-   -   avoidance of flue gas recirculation and, associated with this,         an improvement in the steam generator degree of efficiency by         lowering of the flue gas temperature after the flue gas air         preheater;     -   reduction of investment costs and operating costs for the flue         gas recirculation system;     -   reduction of the convection heating surface size in comparison         with an embodiment with flue gas recirculation.

One advantageous embodiment provides that the platen heating surface which covers part of the enclosure walls in the region of the combustion chamber is arranged between the upper edge of the uppermost burner and the lower edge of the lowermost heating surface. As a result of this measure, a defined region of the combustion chamber is covered by way of a platen heating surface, at which region a large part of the heat would otherwise pass out of the combustion chamber to the enclosure walls and would increase the medium temperature in the enclosure wall and the wall temperature thereof in such a way that a flue gas recirculation system would have to be used in order to reduce the wall temperature.

In one advantageous refinement of the invention, at least part of the enclosure walls is formed from one of the materials T23, T24 or another material with a similar chemical composition. Here, at least that part of the enclosure walls is configured with the abovementioned materials which is thermally loaded highly or more highly than the remaining part of the enclosure walls. The materials T23, T24 or another material with a similar chemical composition are high-quality materials which are commercially available and satisfy the desired requirements or on which no thermal post-treatment has to be carried out after they have been welded.

One advantageous embodiment of the invention provides for the platen heating surface to be formed or produced from martensitic materials with a 9-12% chromium proportion, austenitic materials or nickel-based alloys. This ensures that, with regard to the temperatures, the requirements made of the platen heating surface which lies exposed in the combustion chamber are satisfied.

It is advantageous that the platen heating surface is configured as a superheater heating surface or a reheater heating surface. The platen heating surface is therefore incorporated efficiently into the water/steam circuit of the once-through steam generator or into the water/steam circuit of a power plant which comprises a once-through steam generator of this type.

One advantageous embodiment provides that the platen heating surface is arranged parallel to the enclosure wall. This achieves a situation where the platen heating surface is arranged vertically just like the enclosure wall and affords as small as possible an action surface for ash or cinder from the combustion chamber.

One expedient embodiment provides that the platen heating surface is arranged such that it bears against the enclosure wall. This ensures that the enclosure wall is covered as satisfactorily as possible by the platen heating surface and the smallest possible quantity of heat passes to the enclosure wall.

Exemplary embodiments of the invention are described in greater detail below using the description and the drawing, in which:

FIG. 1 diagrammatically shows a longitudinal section through a once-through steam generator according to the invention, and

FIG. 2 shows the same as FIG. 1, but in an alternative embodiment.

FIG. 1 diagrammatically shows a once-through steam generator 1 (the designation means the generation of the steam within the steam generator in one pass) of tower design, that is to say the tube walls 5 (as enclosure walls 4) and all the heating surfaces 7 are accommodated on or in a single vertical gas flue. The vertical gas flue which is formed or delimited by gastight enclosure walls 4 contains, in its lower region, the combustion chamber 2 and the flue gas pass 3 which adjoins above said combustion chamber 2. The combustion chamber 2 terminates to the bottom as a rule with a combustion chamber hopper and reaches upward as far as the lowermost heating surface 7. One or more burners 6 for burning dry brown coal are arranged in the lower region of the combustion chamber 2. The burners 6 can be arranged either in the corners (corner burners) or in the walls (wall burners) of the combustion chamber 2. The various heating surfaces 7 are arranged as convection heating surfaces in the flue gas pass 3. Said heating surfaces 7 are as a rule economizer heating surfaces, superheater heating surfaces and reheater heating surfaces. The flue gas pass 3 terminates at the top with a ceiling and has a flue gas outlet 9 laterally at its upper end.

According to the invention, the once-through steam generator 1 has at least one platen heating surface 8 which covers part of the enclosure walls 4 in the region of the combustion chamber 2 and the surface-side size of which is defined such that the heat absorption of the enclosure walls 4 and, as a consequence, their temperature are reduced to a value which permits the configuration of the enclosure wall 4 from modified, heat-resistant 2.25-2.5% chromium steels which do not require any thermal post-treatment after they have been processed using welding technology. In other words, the platen heating surface 8, which covers the enclosure wall 4 in the region of the combustion chamber 2 with a predefined surface-side size, absorbs so much heat from the combustion chamber 2 that the heat absorption of the enclosure wall 4 is reduced as a consequence of the covering, in such a way that the maximum medium temperature at the enclosure wall 4 remains below a value which allows the use of modified, heat-resistant 2.25-2.5% chromium steels which do not require any thermal post-treatment after they have been processed using welding technology. They can be, for example, the materials T23 (a material which is approved by the ASME (American Society of Mechanical Engineers)), T24 (7CrMoVTiB10-10) or another material with a similar chemical composition which can cover steam temperatures up to approximately 500-510° C. and which are listed, for example, in the brochure “The T23/T24 Book, New Grades for Waterwalls and Superheaters from Vallourec & Mannesmann Tubes” (brochure about modified, heat-resistant 2.25-2.5% chromium steels). As a result of the reduction in the medium temperature in the enclosure wall 4 by means of the platen heating surface 8 which is arranged according to the invention, the use of flue gas recirculation systems can be dispensed with, which systems otherwise conduct cooler flue gas into the combustion chamber 2 and therefore reduce the temperatures within the combustion chamber 2 and, as a consequence, also the medium temperature in the enclosure wall 4.

These abovementioned high-quality materials which do not require any thermal post-treatment after they have been processed using welding technology can either be used everywhere on the enclosure wall 4 or, according to one commercially more advantageous variant, at least at those parts of the enclosure walls 4 where the high thermal loading makes it necessary. They are, for example, the regions at the burners 6 and directly above the burners 6 within the combustion chamber 2. In order to reduce the investment costs in comparison with the abovementioned high-quality materials, lower-quality materials, such as 16Mo3 or 13CrMo45, are used at those parts of the enclosure walls 4 where the thermal loading is lower, such as in the lower part of the combustion chamber 2 (below the burners 6 including combustion chamber hopper) with medium temperatures of approximately ≦400-460° C. in the tube walls. Said materials likewise do not require any thermal post-treatment after they have been processed using welding technology.

The enclosure walls 4 which are configured as tube walls 5 are produced as a rule from a welded tube-web-tube combination, the tubes of the tube walls 5 conducting the working medium water/steam, and it being possible for them to be formed within the enclosure walls 4 either helically or vertically or from a combination of helically and vertically. The tubes which are arranged in the enclosure walls 4 are used in the lower and middle part of the combustion chamber 2 as evaporator tubes, that is to say the water which is fed in and pre-heated is evaporated in said evaporator tubes. In the upper part of the combustion chamber 2 which has perpendicular tubes as a rule, the tubes which are arranged in the enclosure wall 4 can already be connected as a superheater heating surface.

The platen heating surface 8 itself, which then absorbs part of the heat from the combustion chamber 2, is formed using suitable materials according to the temperature requirements. Since very high temperatures have to be handled, martensitic 9-12% chromium-containing steels, austenitic steels or nickel-based alloys have proven to be suitable for this purpose. They can be, for example, the martensitic materials T91 (X10CrMoVNb9-1), T92 (X10CrWMoVNb9-2) or VM12-SHC, the austenitic steels SUPER 304H, HR3C, DMV304HCu, DMV3101N or nickel-based alloys such as Alloy 617 (NiCr23Co12Mo) or Alloy 617mod (NiCr23Co12Mo mod). The platen heating surface 8 can consist of individual tubes, which are arranged close to one another and in parallel, or of a tube-web-tube construction. The tubes of the platen heating surface 8 run as a rule horizontally within the heating surface, but can also run vertically.

The platen heating surface 8 is preferably arranged parallel to the enclosure wall 4 and more preferably so as to bear against the latter. This arrangement ensures that the enclosure wall 4 is covered very efficiently by the platen heating surface 8, and the transmission of heat to the enclosure wall 4 is therefore suppressed as far as possible. FIG. 2 shows one advantageous variant of the platen heating surface 8 according to the invention. Here, the enclosure wall 4 and tube wall 5, which as a rule contain the front and rear walls and two side walls of the once-through steam generator, are covered partially by one or more platen heating surfaces 8 in the region of the combustion chamber 2, to be precise between the upper edge of the uppermost burner 6 and the lower edge of the lowermost heating surface 7 (the region is marked or denoted by “S” in FIG. 2), one platen heating surface 8 being arranged on each individual tube wall, that is to say a total of four, according to FIG. 2. As a result of the targeted arrangement of the platen heating surface 8 specifically in this region of the combustion chamber 2, that region of the enclosure wall 4 or tube wall 5 within the combustion chamber 2 which is hottest as a rule can be covered in a very targeted manner. The platen heating surface 8 can advantageously be used as a superheater heating surface within the once-through steam generator 1. However, the use as a reheater heating surface is also possible.

LIST OF DESIGNATIONS

-   1 Once-through steam generator -   2 Combustion chamber -   3 Flue gas pass -   4 Enclosure wall -   5 Tube wall -   6 Burner -   7 Heating surface -   8 Platen heating surface -   9 Flue gas outlet 

1. A once-through steam generator for burning dry brown coal without the aid of recycled flue gas in its combustion chamber, the once-through steam generator (1) having a combustion chamber (2), a flue gas pass (3) which adjoins the upper end of said combustion chamber (2), and enclosure walls (4) which enclose them, the enclosure walls (4) being formed from tube walls (5), the tubes of which conduct the working medium water/steam, the combustion chamber (2) having at least one burner (6), and heating surfaces (7) being arranged in the flue gas pass (3), part of the enclosure walls (4) being covered in the region of the combustion chamber (2) by at least one platen heating surface (8), the surface-side size of which is defined such that the heat absorption of the enclosure walls (4) and, as a consequence, their temperature are reduced to a value which permits the configuration of the enclosure walls (4) from modified, heat-resistant 2.25-2.5% chromium steels which do not require any thermal post-treatment after they have been processed using welding technology.
 2. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) which covers part of the enclosure walls (4) in the region of the combustion chamber (2) is arranged between the upper edge of the uppermost burner (6) and the lower edge of the lowermost heating surface (7).
 3. The once-through steam generator as claimed in claim 1, characterized in that at least part of the enclosure walls (4) is formed from one of the materials T23, T24 or another material with a similar chemical composition.
 4. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) is formed from martensitic steels with a 9-12% chromium proportion, austenitic steels or nickel-based alloys.
 5. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) is configured as a superheater heating surface.
 6. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) is configured as a reheater heating surface.
 7. The once-through steam generator as claimed in claim 1, characterized in that the platen heating surface (8) is arranged parallel to the enclosure wall (4).
 8. The once-through steam generator as claimed in one of the preceding claims claim 1, characterized in that the platen heating surface (8) extends such that it bears against the enclosure wall (4). 